Engines may use various combustion modes across different operating conditions to improve overall engine performance. For example, spark ignition (SI) combustion may be used under some conditions, while controlled auto-ignition (CAI), or homogenous charge compression ignition (HCCI), may be used under other conditions. In some cases, operating in these different modes uses different valve timing and/or lift. The different valve operation may enable the engine to reliably achieve the desired type of ignition or combustion while meeting emissions and performance targets.
One approach to provide different valve operation in different combustion modes is described in U.S. Pat. No. 6,640,771. In this example, multiple intake valves are operated with different lift profiles in different combustion modes. Further, in one example, at least two intake valves are provided in each cylinder, where one intake valve is changeable between a first and second lift and the other intake valve may be actuated into an OFF position by means of a valve cut-off device. Thus, each intake valve switch changes an effective valve lift in different modes.
The inventors herein have recognized disadvantages with such an approach. In particular, because each valve has switchable valve operation in order to provide the different lift profiles, and there are multiple valves per cylinder, each cylinder may need multiple actuators. In multi-cylinder engines, such increased numbers of actuators can result in increased system cost and weight. Further, the increased number of actuators may require increased hydraulic pressure or capacity in the case of hydraulic actuators, thus potentially further increasing weight and increasing cost.
In one approach, the above issues may be addressed by a method for operating an engine of a vehicle, the engine having a cylinder with at least a first and second valve, the method comprising: during a first mode, operating with the first valve active and the second valve deactivated during a cycle of the cylinder, where during said first mode, the cylinder operates to allow at least air to enter the cylinder during an intake stroke, where the air is mixed with fuel and compressed to attain auto-ignition; and during a second mode, operating with the first and second valves active during a cycle of the cylinder, where during said second mode, cylinder operates to allow at least air to enter the cylinder, where the air is mixed with fuel and ignited via a spark from an ignition plug.
In this way, it is possible to provide a transition between different lift profiles when changing combustion from auto-ignition or compression ignition to spark ignition by simply activating the second valve (although changes in other valve operation may also be used, if desired).